1. Field of the Invention
The present invention is directed to silicate based glass compositions with phosphorus compounds included in the composition as spectral modifiers and the methods of making the composition and glass articles formed therefrom. More particularly, the present invention is directed toward the use of metal phosphides in silicate based glasses for the improvement of spectral properties such as the absorption of infrared energy and the transmittance of color. Certain metal phosphides, specifically iron and zinc phosphides, significantly improve the absorption of near infrared radiation in silicate based glass compositions.
2. Summary of Related Art
Spectral modifiers are often added to base glass compositions to impart specific color and energy absorbance properties in the finished glass. The absorption of energy at specific wavelengths is often desirable to enable various uses for the glass compositions. Additionally, certain colors are preferred for various glasses for aesthetic reasons. There are limitations within specific glasses that must be balanced or optimized when attempting to achieve desired color and energy transmittance properties. For example, certain ingredients may improve the absorption of near infrared energy while imparting an undesirable color or reducing the light transmittance. Thus, the optimization of a specific color or energy transmittance property often negatively impacts other desirable transmittance properties.
The use of phosphorus, measured as P.sub.2 O.sub.5, as a network former is known in the art for producing phosphate glass compositions. A network former is the primary cation in the glass composition that bonds with oxygen to create the amorphous network. Thus, phosphate glass compositions are generally glasses having over a 25 mole % P.sub.2 O.sub.5 content. Other cations, such as silicon, may be included in a phosphate glass composition. However, a phosphorus content in excess of the noted limit is generally classified as a phosphate glass. Phosphorus is typically introduced to the glass batch composition in the form of phosphate salts. It is generally understood in the art that the viscosity of molten phosphate based glass compositions is not suitable for application in a float bath process. Additionally, phosphorus, at elevated levels, can adversely affect the brickwork in the float glass process.
Phosphorus based glasses often have desirable infrared or heat absorbing capabilities. U.S. Pat. No. 3,944,352 discloses a heat absorbing glass composition having acceptable visible light transmittance and heat absorbing capabilities. The glass composition contains 50-65% P.sub.2 O.sub.5, with 1-9% of the P.sub.2 O.sub.5 added in the batch as ammonium phosphate. The glass exhibits a visible light transmittance in excess of 65% and an infrared transmittance of below 30%. Although the optical properties of the glass are desirable, the physical properties and manufacturing requirements of a phosphate glass prevent them from being manufactured in a float glass production process.
Phosphates have also been included in silicate based glasses, wherein silicon is the primary network forming cation. For example, U.S. Pat. No. 4,713,359 discloses the use of phosphates, measured as P.sub.2 O.sub.5, in a silicate based glass composition. The composition uses up to 1% by weight of P.sub.2 O.sub.5. The resulting glass has a visible light transmittance (Ill A) of greater than 70% and total solar heat transmittance of less than 50%. In general, phosphates in silicate glasses are added in the salt form (PO.sub.4.sup.-3) in amounts less than about 3 wt %. Phosphate contents of greater than 4 wt % generally result in phase separation in the glass.
Infrared absorbing, or heat reducing, silicate glasses are also known within the art. In general, infrared absorbing silicate glasses involve the addition of specific colorants that impact the color and energy transmittance properties of the glass. It is generally known to manufacture heat or infrared radiation absorbing silicate glass by the incorporation therein of iron. The iron is generally present in the glass as both ferrous oxide (FeO) and ferric oxide (Fe.sub.2 O.sub.3). The balance between ferrous and ferric oxide has a direct and material effect on the color and transmittance properties of the glass. As the ferrous oxide content is increased (as a result of chemically reducing ferric oxide), the infrared absorption increases and the ultraviolet absorption decreases. The shift toward a higher concentration of FeO in relation to the Fe.sub.2 O.sub.3 also causes a change in the color of the glass from a yellow-green to a blue-green, which reduces the visible transmittance of the glass. Therefore, in order to obtain greater infrared absorption in glass without sacrificing visual transmittance, it has been deemed necessary in the prior art to produce glass with a low total iron content which is highly reduced from Fe.sub.2 O.sub.3 to FeO.
U.S. Pat. No. 4,792,536 discloses a process for producing an infrared energy absorbing glass, containing a low total iron concentration which is highly reduced to FeO. It is further disclosed that the infrared energy absorption can be increased by including greater amounts of total iron in the glass composition, but states that the visible light transmittance would thereby be reduced below levels considered adequate for automotive glazings. The disclosed process utilizes a two stage melting and refining operation, which provides highly reducing conditions so as to increase the amount of iron in the ferrous state for a given low total iron concentration of from 0.45% to 0.65% by weight. The patent teaches that the iron must be at least 35% reduced to FeO.
Another example of an infrared absorbing silicate glass is found in U.S. Pat. No. 5,077,133. The patent discloses a green colored infrared and ultraviolet absorbing silicate glass which includes an amount of ceric oxide, or alternatively ceric oxide and titanium dioxide, and a high concentration of moderately reduced iron. The glass composition exhibits a visible light transmittance of at least 70% and a total solar energy transmittance of less than 46%. Although the glass composition exhibits a low solar energy transmittance, it is desirable to further reduce the total solar energy transmittance, through the absorption of near infrared energy, while maintaining the high visible light transmittance.
It would be an advantage to provide a spectral modifier for use in a silicate glass composition that has the desired color and significantly improves energy absorbance properties without adversely impacting other transmittance properties.
It would also be an advantage to utilize a phosphorus compound in a silicate based glass composition at a level sufficient enough to impact the spectral properties without causing phase separation in the finished glass composition.
It would be a further advantage to increase the absorption of infrared energy without adversely affecting the visible light transmittance of the finished glass composition.
It would also be an advantage to utilize a spectral modifier to improve color and energy properties in a silicate based glass composition wherein the inclusion of the spectral modifier does not adversely impact the manufacturing of the glass in a float glass production process.